Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2022 / Paper 1 / Section B / 10 / vii / Solution

The chemically supported assignment isThe four oxygen donors can converge around the small lithium ion to organize the alkene ends efficiently. The larger sodium and potassium ions are less well matched to this particular folded four-oxygen binding environment, with potassium giving the weakest enhancement in the stated comparison. This is a match between donor geometry, solvation and cation size, not a universal rule that lithium templates every crown ether best.

The primary chemical record supports the identity assignment: the inventors' Table 1 reports a strong lithium enhancement for the 14-membered product, with much smaller effects from sodium and potassium. It lists the potassium yield as $36\%$, whereas this examination gives $32\%$; that discrepancy should not be hidden by treating the exam numbers as a verbatim reproduction. The ion assignment follows the structure and the documented templating comparison, rather than the numerical yield alone.^[ref]